1. Field of the Invention
The present invention relates to a magnetoresistive element, i.e., an element that changes its resistance according to an applied external magnetic field (magnetoresistive effect), and to a method for making the same. The present invention is also directed to a magnetic memory device including the magnetoresistive element.
2. Description of the Related Art
Recently, magnetic random access memories (MRAMs) have drawn much attention among the magnetic memories that function as memory devices. Magnetic random access memories use magnetoresistive elements of a giant magnetoresistive (GMR) type or a tunneling magnetoresistive (TMR) type and store information using the magnetization rotation of the magnetoresistive elements.
For example, a TMR element of an MRAM includes a free layer composed of a ferromagnetic material, a nonmagnetic intermediate layer composed of an insulator, a pinned layer composed of a ferromagnetic material, and an antiferromagnetic layer for directly or indirectly pinning the magnetization direction of the pinned layer. These layers are stacked in that order so that the resistance of the tunneling current can change according to the magnetization direction of the free layer. In the MRAM, “1” is recorded when the magnetization of the free layer is oriented in a particular direction, and “0” is recorded when the magnetization of the free layer is oriented in the opposite direction. In reading the information recorded in the MRAM, the magnetization direction of the free layer is detected as the voltage signal generated by the change in resistance of the tunneling current.
Magnetoresistive elements of MRAMs are becoming increasingly smaller to increase the degree of integration. Naturally, free layers having rotatable magnetization where switching operation is performed are also becoming smaller. However, in small free layers, the distance between two ends of the free layers, i.e., the distance between magnetic poles of the free layers, is also small, resulting in an increase in demagnetizing field in the free layer. Since the demagnetizing field decreases the intensity of the external magnetic field applied to the free layer, the coercive force of the free layer is significantly increased, and the magnetization of the free layer can no longer rotate to perform switching operation unless a larger magnetic field is applied to the free layer. In other words, with an increased demagnetizing field, the current supplied to the electrode layer for rotating the magnetization direction of the free layer must also be increased. As a result, the energy consumption required for writing information increases.
One way of preventing an increase in coercive force due to the demagnetizing field is to decrease the moment of the free layer to make the demagnetizing field less dependent from the size of the element. Here, the moment is the product of the saturation magnetization Ms of the ferromagnetic material constituting the free layer and the thickness of the free layer. This is possible because the relationship Hd=A×Ms×t/W is established among the demagnetizing field Hd, the moment Ms×t, and the length W of the element in the direction of application of the magnetic field (usually the easy-axis direction). Changing the ferromagnetic material constituting the free layer is difficult since such a change significantly affects the magnetoresistance (MR) ratio; thus, the moment of the free layer must be decreased by reducing the thickness of the free layer.
However, at an excessively small thickness, e.g., a thickness of approximately 1 to 2 nm, the free layer may not be formed as one continuous layer or may suffer from degraded heat stability. An increase in coercive field due to the demagnetization cannot sufficiently be prevented by merely reducing the thickness of the free layer.
FIG. 6 is a graph showing the dependency of the saturation magnetization of a NiFe layer on the layer thickness in a composite prepared by stacking a tantalum (Ta) layer, a nickel-iron (NiFe) layer, and another tantalum (Ta) layer. As is apparent from the graph, the saturation magnetization of the NiFe layer decreases dramatically below a certain thickness, e.g., approximately 2 nm. This is because a free layer having an excessively small thickness has, for example, an island structure, no longer forms a continuous layer, and suffers from thermal diffusion from adjacent layers. Although a decrease in the saturation magnetization can contribute to making a free layer having a low moment, the moment significantly varies according to minute changes in the layer thickness within the range of approximately 1 nm, resulting in poor reproducibility and large variation. This also poses a limit to thickness reduction of the free layer.